An asymmetric azine compound shown by the following formula (A) has been known as a liquid crystalline compound.
wherein Ra represents an alkyl group, and Rb represents an alkyl group, a cyano group, a fluorine atom, a trifluoromethoxy group, or the like.
The above asymmetric azine compound is a liquid crystal material that shows a liquid crystal phase over a wide temperature range, is relatively chemically stable, and can be produced inexpensively, for example.
The above asymmetric azine compound may be produced by (i) reacting a first aldehyde compound with hydrazine to prepare a hydrazone, and reacting the resulting hydrazone with a second aldehyde compound (Patent Document 1), or (ii) reacting an aldehyde shown by the following formula (B) with a large excess of hydrazine in an alcohol solvent to obtain a reaction solution including a hydrazone compound shown by the following formula (C),
wherein Ra is the same as defined above,
wherein Ra is the same as defined above, isolating the hydrazone compound shown by the formula (C) from the reaction solution, and reacting the hydrazone compound with an aldehyde shown by the following formula (D) to obtain an asymmetric azine compound shown by the formula (A) (Patent Document 2), for example.
wherein Rb is the same as defined above.
However, when producing an asymmetric azine compound that includes a polar group (e.g., hydroxyl group or carboxyl group) in the molecule, since the hydrazone compound (intermediate) has high water solubility, it may be difficult to isolate the hydrazone compound from the reaction system. This makes it difficult to directly apply the method disclosed in Patent Document 1 or 2.
Patent Document 2 refers to a problem that may occur when using the method disclosed in Patent Document 1. Specifically, Patent Document 2 describes that the target product may not be obtained in high yield due to a disproportionation reaction when using the method disclosed in Patent Document 1.
It is known that a disproportionation reaction is promoted by an acid. Specifically, a disproportionation reaction more easily progresses when a polar group (e.g., hydroxyl group or carboxyl group) is present in the molecule. Therefore, it is difficult to produce an asymmetric azine compound that includes a polar group (e.g., hydroxyl group or carboxyl group) in the molecule in high yield.
This problem may be solved by utilizing an aldehyde compound in which a polar group is protected by an appropriate protecting group. In this case, the reaction selectivity can be improved. However, this method is not industrially advantageous from the viewpoint of production cost due to the use of an expensive protecting group.